Trouble fitting hatch ribs to top & bottom hatch seal rails

[Mr. Lahey]

I could really use some advise. Or this is going to take me a long time to figure out. This post might be photo intense as I try and describe the trouble I'm having

Right now I have the upper, lower, and left and right sides of my hatch frame fit into the galley opening. I am now trying to fit 4 ribs across and getting it wrong.

The ribs are birds mouthed at the top side to connect to the underside of the top hatch seal rail. The they would be flush cut to the lower vertical seal rail. I made the right side rib first. Fit it. Then used it as a pattern to make the left. It all went great. As I check side to side with a long straight edge they are in plane with each other through the whole curve of the hatch from top to bottom.

Hatch rib that was used as pattern

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I have 2 spare ribs to trial fit with. When I use the same rib for a pattern I flush up the surface of the rib with the pattern rib. Make sure it's flush through out whole curve of the rib. Then I trace the birds mouth at the top of rib, and cut it into the trial rib. That goes great and fits well at the top. The bottom of the rib needs to lay flush/flat on the lower seal rail. The side rails do not attach that way. they carry on past the rail as you can see in the photo below.

Side rib carries past seal rib and weather strip spacer. Rib attaches to side of seal rail not face.

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When I cut the trial rib at point shown by pencil in photo below.....it ends up short at the bottom rail.

Where I cut center rib

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[Mr. Lahey]

Photo below shows how short rib ends up after cutting. The rib also sinks in after the high spot of hatch curve. When using the straight edge across both sides of galley the center rib stays in plane with side ribs till I get to about the high point of the curve. Then it sinks away from the straight edge quickly as it curves back to the bottom as seen in second photo.

Rib short of bottom edge of seal rail after cutting to fit flush on face of rail

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Bottom of hatch curve-Gap at straight edge from center rib out of plane

DSCF3806a.jpg (131.87 KiB) Viewed 2661 times

When I add back a space block the same thickness of the seal rail, the rib will be in plane with side ribs from the bottom of the hatch to the high point. From the high point of the curve going up to top tail the rib is too tall and is above plane of the side ribs.

What am I doing wrong here?

[lincolnlerner]

I am not clear on how you re measuring where to cut the lower end to the middle ribs. Seems you are putting it next to the side rib and everything matches. Cutting top to fit the hard points etc. Then does it still match the side rail or the master rib?

otoh If a block solves all issues, length, position curves.... then it would appear to be a simple singular non-systemic issue. An issue after all.
If I am not understanding the way they are matched up and then measured for cut....

[pchast]

I'd encourage you to take a step back and check all rails for straight and the hatch for square.
You need to repeat measurements at the expected rib locations top to bottom. You should find
the changes. You may need to project the plane of the hatch cover and measure or mark a hatch
board as in fitting for boat interiors.

Next, I see no shims between the side ribs and the walls. If you make it that tight it will not
close well when completed.

[troubleScottie]

So looking more closely at DSCF3807a.jpg, I see that the right (center?) rib does not go all the way down to the bottom of the horizontal wood strip.

One solution is the the cut out on the rib needs to extended up so the rib will settle more. I cannot set the other side of the rib in the picture.

Two is to carefully inspect the opening.

Is the top and bottom edges of the hatch perpendicular to the walls. Both edges have to be straight AND the same length. That includes the two strips of wood that you are using to be the bottom of the hatch. The walls must be vertical also. With bows, up and down, etc, the ribs will not match. You should be able to compare the edge to a long level or a metal guide strip -- 36"+ inches to confirm it is straight along both y and z axis. If this is true, then use a speed square or construction square to compare it to the walls. They should be the same for both walls. Repeat the process to confirm the walls are straight and perpendicular to the floor. Finally measure the diagonal. This is redundant, but you cannot be too careful.

Another test is to measure the top edge to the bottom edge at the rib placement position. They should be the same.


If it is not square, you have to make adjustments. You could rebuild the opening OR build a square hatch and fit it to the opening OR custom draw/cut the ribs to correct for the errors.

[Mr. Lahey]

okay to answer a few questions:

1. Yes-the hatch opening is square. The top forward edge of hatch frame is equal distance from the floor across the opening.

2. I am as perpendicular/vertical/ level/square with galley walls, floor, etc. as humanly possible.

3. There is a 1/4" gap from wall to side ribs. I just do not have spacers shoved in the gap. But its there.

4. The top frame rail is equal distance from bulkhead face across entire width of opening.

5. All six ribs, from left to right were clamped together and belt sanded to same shape across the skin contact surface of ribs. When clamped together the skin mount surface is as smooth and flat as can be across all six ribs.

When I try to make a 3rd rib to match the 2 side ribs, the top side fits fine. The birds mouth fits the top rail. The new 3rd rib stays in plane from the top of the hatch to the high point of the hatch curve. As I go further down the hatch curve with the straight edge, once I get near the hatch curve high point the new 3rd rib starts to move away from the straight edge and open up a gap. The gap gets worse as I follow the curve to the bottom. See below:

Top of new 3rd rib in plane with both side ribs per straight edge.

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Straight edge gap at new 3rd rib at bottom of hatch curve near lower seal rail

DSCF3806a.jpg (131.87 KiB) Viewed 2607 times

The two side ribs go PAST the lower seal rail and screw into the side or end grain of the rail. The four new center ribs will need to mount on the FACE surface of the lower seal rail. They will mount to the rail by being screwed to the face of the rail from the back side of the rail.

The pictures below again best shows how, where, and why,I am cutting the 3rd rib. In theory if I cut the new 3rd rib along the pencil line where the pencil is pointing, that should allow the end of the new 3rd rib to sit right on the FACE of the lower seal rail in photo. The rib you see in this photo is the existing side rib. The side rib goes past the rail as shown and screws into the side of the rail not the face.

When I cut the new 3rd rib where it should in theory rest on the face of lower rail it comes up short. As shown at top of this post. Yet it fits fine up top. New 3rd rib in plane as I go down the hatch curve. Till about the curve high point. Then it begins to gap away from the straight edge the further down the curve I go.

Pencil points to where I am cutting bottom for 3rd rib

DSCF3809a.jpg (111.88 KiB) Viewed 2607 times

I think I've spent more time trying to explain the issue than solve it..

[lincolnlerner]

Okay sure its not upside down.
Middle ones perfectly match before cutting. You are not making the top cut the same as the two sides or wxactly the same. Should they be? With a block at the bottom does it work out.

[lincolnlerner]

Read the post again and studied pictures. Is it possible to make a cardboard or hard paper copy and test that without cutting the top to the hinge board. Just the bottom. Or matching it up on the outside making it a bit easier to see where the issue is. Yes withoit forming to the top, follow curve up. See where issue is. Hingeboard may not be perfect

[Mr. Lahey]

Thank you all for the replies. I have come up with a plan of attack to solve this issue. Here is what I'm going to try.

Going to clamp 4 straight edges across the opening. Near top attach point, mid way down to curve high point, at curve high point, and bottom of curve.

Then I will test the side rib I used as a pattern to make the ribs, at each of the 4 locations that the new center ribs will attach too.
If the pattern rib can touch all 4 straight edges, at the 4 places the new ribs will attach to, I will then cut my ribs to exact match the pattern rib.

I think until a new rib can touch the 4 straight edge points, only then can it be marked for where to cut bottom attach point of rib.
I know for a fact the test rib I cut in this post could not pass this test. Somehow I got something out of wack when I first positioned the test rib and cut it.
This method should eliminate that from happening again.

I'll stop back and post here, if this method gets me back on track again.

[KCStudly]

Is it possible that cutting just to one side of the pencil line would be enough to throw it off that much? The notch at the top might not feel the difference, but a 'shave and hair cut' at the bottom would quickly show.

[Mr. Lahey]

Had some time after work to try out my 4 straight edge theory to locate the new center ribs. So I picked up some lumber to use for straight edges on the way home, and gave it a whirl. I think this is going to do it.

First thing I did was build a small jig to simulate the size and location of the lower seal rail I want to attach the ribs to. Then I removed the right side inner galley wall rib I used as a pattern for the left side inner rib. I clamped that pattern rib to the jig and test fit that rib at all 4 locations for the new ribs. Sure enough the sample rib touches all 4 straight edges. It stays in plane with both outside ribs. It also is right down to the bottom of of the lower rail as it's clamped to the jig.

DSCF3812a.jpg (112.21 KiB) Viewed 2467 times

So based on this I'm going to cut the bottom of the new ribs as shown. This time I think they will fit right. Thanks all for the help thinking this through guys.

DSCF3813a.jpg (114.54 KiB) Viewed 2467 times

[Aguyfromohio]

Glad you are finding a solution. We too had to fuss around placing and trimming the ribs while clamped in place, looking at everything with straight edges and strings. It's a truly tree-dimensional geometry problem, with every component affecting all the others. And of course we got some spring-back from the skins so the hatch is not exactly the same shape we intended, the skins pulled the curved ribs straight a bit. We now have a better appreciation for the approaches that build the hatch glued solid and complete onto the walls and then cut it away last. It seems a pretty smart way to go.

Placing and sizing the gas spring struts for the galley hatch was that way too. Here's our take-away on the gas springs: Everything is easier to fit up when the gas spring attachment points are closer to the hinge. But the closer the gas strut gets to the hinge, the higher the forces need to be for the gas strut to lift the hatch. So build the hatch light and make it extra strong near the hinge to take the forces. We ended up with the longest gas struts commonly available, 44 inch length fully open with 65 lbs of force, and had a lot of trouble finding a mounting geometry that worked both open and closed. Especially closed. Next time we will build a lighter hatch and use short little gas springs about 20 inches long with big forces of about 150 lbs, and we'll make the galley hatch strong enough at the attachment points to withstand those forces. Just like the car makers do for hood and trunk lift supports.

[Mr. Lahey]

Came back for a little follow up on how it's going.

The 4 straight edge thing worked to the point of getting me in the ball park. However it wasn't seats behind home plate. As KC and AGuy said minute changes or errors get magnified QUICK. KC was right that even cutting on one side of a line or the other made a big difference in how the rib finally fit at it's best. Unbelievable how 1/16th's of an inch would affect the whole path of the rib around the curve. There is a definite multiplier effect working with these curves.

I have 2 of the 4 center ribs cut and fit with considerable time into each. I just came in now to have dinner and give it a rest. As I fit those 2 ribs I will say I agree with AGuy 100%. Make sure EVERYTHING is as dead on as you can get it. Like rib spacing, location, squareness, before you final fit. The least movement from dead on in any direction, shows up right away when you straight edge across the width of the opening. Plus if you don't get this right the plywood skin will follow the mistakes in the ribs and be wavy or have a weird hump to it as you sight across the hatch.

I have a much much better understanding of why many builder use horizontal ribs in the hatch instead of vertical! So I'm half way there, and looking forward to getting past this part of the project.

Thanks for the tip on the gas struts. I was wondering about that too. I may heed your advice and go with the shorter higher pressure ones.

[noseoil]

I cut all of the ribs at the same time on my build & used a cross-piece at top & bottom (like a regular frame wall build in a house). Much easier that way.

Layout for the strut hardware was done first, on a full scale cardboard mock-up of the galley & hatch. Just three swings of a compass for the arcs will give you fastener locations & mounting points, once you have plotted the hatch in open position, a hinge point & the closed position. Don't forget to have the hardware offsets you are using as well. For the closed position, use a 1/2" shorter arc than the actual strut length so you don't create a bind as it swings shut.

[Mr. Lahey]

I think I grasped 60% of what you said about the three compass swings for the struts. Sounds like the way to go.

I might ask if you could break it down a tad further for me to get the other 40% of the concept down pat.

Something like

1. Arc one- With hatch open set compass at ______ inches open and locate compass point at ______ and draw arc to touch ______?

2. Arc two- with hatch close set compass at________ inches open and locate compass point at_________ and draw arc to touch_______?

Be a huge help to me and I'm sure a few others.

Thanks!!